Although this issue might be more severe for Lemmy, I think it still occurs on Reddit, but it ends up working itself out. For instance, there's also a "r/tech" and (at some point) there was a "r/technologynews" subreddit, which you could view as redundant and possibly confusing to new users. But r/tech seems to have focused on longer-form more in-depth discussion while r/technology is certainly the larger and more general of the two, so most users will probably just sub to the larger one and maybe others will decide they like the vibe of r/tech more. In the long term, I'm sure a similar thing will happen to communities with the same, or close to the same, name.

Certain communities seem to be inaccessible from Lemmy.world and vice versa. For instance, if I try to access /c/boston in Lemmy.ml via lemmy.world/c/boston@lemmy.ml, I get a "404: couldnt_find_community" error. Similarly, trying to access /c/boston in Lemmy.world from either Kbin.social (via kbin.social/m/boston@lemmy.world) or Lemmy.ml (via lemmy.ml/c/boston@lemmy.world) both give 404 errors.

Yeah possibly! I think the second issue I mentioned is still occurring, though. Namely, lemmy.ml/c/boston@lemmy.world returns a 404 error (and also for Kbin.social).

Perhaps! This should be triggered by searching "!boston@lemmy.world" from lemmy.ml, correct?

I think you're totally right! That's partially why I said it's a bigger problem on Lemmy. Tools like browse.feddit.de will be pretty important; it'd be great if they were placed prominently somewhere.